A Detailed Introduction about Packet Classification and Tagging

Updated: Jul 2

HCIE Routing and Switching Interview Question tips

Quality of Service is used to evaluate the ability of servers that they can satisfy the service command of customers. Through installing the QoS, they can adjust and control the network flow of enterprises, avoid and manage the network jam, and reduce the rate of message loss. Meanwhile, they can provide the enterprises’ users with the dedicated broadband, or differential service for different businesses ( voice, video, data and so on). During the HCIE interview in data communication, we may face the examination questions related with Quality of Service. Huawei Certificated Expert Knowledge Column will comprehensively and entertained analyze the QoS through three series of papers, in order to make people easily absorb the related skills with QoS. First, we will take you to have an understanding about the Packet classification and tab.

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The Introduction of QoS

We will use a picture to introduce QoS, which shows the complete framework and logic of QoS.

At present, the Quality of Service is often applied into the Differential service model. But how many models does QoS have?

 Best Effort model: being indirect, having no special treat towards others and transmitting message for the first one.

② Integrated Service model: It will use RSVP Agreement to reserve one for a sole usage. For example, it will reserve a broadband with ten megohm special flow for customers. The merit is that it can make a difference between special flow. While the weakness is that the broadband with ten megohm flow can’t be used in other fields when customers do not use it, which will waste the broadband.

③ Differentiated Service model: we will focus on introducing it.

PS: If you faced the QoS when being interviewed, you should state the three models in order. When it came to the Differentiated Service model, you could carry on the below content smoothly.

But how does the Differentiated Service model make a difference without wasting the broadband? At first, we can take an access from the data, voice and video flow, and then we will classify and tab from one to one. However, how do you classify them? And how do you make a tab? Where do you tab? How do the exchanger and Routing and Switching distinguish the flow of data, or voice, or video?

Packet Classification and Tagging

There are two ways to tagging. The one refers to a simple one, and regarded as simple flow classification, which uses the fields tabbed in VLAN, MPLS and IP to make a classification. The other refers to complex flow classification, which uniquely distinguishes a flow through 5 forms such as source IP, destination IP, source port, destination port and transport layer protocol.

After the classification, it is tab, which can be labeled in three places.


1. PRI fields in VLAN tag (Here we need to remember the form and length of VLAN)


2. EXP fields in MPLS


3. TOS fields in IP Precedence

The amount of IP Precedence are three bits, so the priority level can be defined from 0 to 7. Without enough usage, there develop D,T and R, named as DSCP. Thus, the priority level being defined is from 0 to 63. The form three are called IPP, while the former five are named as DSCP.

Now voice, data, and video flow classify and tabbed the precedence according to the rules of IP priority or DSCP priority, input interface, IP packet quintuple, etc. Before entering the exchanger or Routing and switching, we can make a flow management. And we will not talk about this part, but talk about it in detail later. Then, when it enters into the exchanger or Routing and Switching, there is a virtual queue inside. That the high priority will enter into the high queue, as the low priority to the low queue. So which protocol does define our tabbed priority 7 being better than priority 0? It is a PHB (Per Hop Behavior) protocol, which summarizes the role of this protocol in one sentence. That is Defining Service level, i.e 7>6>5>4>3>2>1.

Next there is a format, which helps us know more about PHB protocol.

The PHB protocol makes a definition about queues including the CS, EF, AF and BE. We can understand it from this way. That A, DSCP has 63 priorities from 0 to 63 in all. B: IPP, MPLS and VLAN have seven priorities in total from 0 to 7. Let’s divide it and call DSCP as A, while IPP, MPLS and VLAN are named as B.

You can carefully observe the chart and you will find the relation among the A-B-CS. It is a truth that PHB concludes the priorities into CS. While what are EF, AF and BE?

BE is a special name when DSCP is 0 in CS0. (If it is tabbed as 0, it will enter into the BE queue and do its best to transmit.)

EF is a special name when DSCP is 46 in CS4.(If tabbed as 46, it will enter into FE queue and be fast transmitted.)

AF1 is a special name when DSCP is 10, 12 and 14 in CS1. (If tabbed as 10, 12 and 14, respectively being AF11 queue, AF12 queue and AF13 queue, it will enter into the AF1 queue and be assured to transmitted.)

AF2 is a special name when DSCP is 18, 20 and 22 in CS1. (If tabbed as 18, 20 and 22, respectively being AF21 queue, AF22 queue and AF23 queue, it will enter into the AF2 queue and be assured to transmitted.)

AF3 is a special name when DSCP is 26 or 28 or 30 in CS1. (If tabbed as 26, 28 and 30, respectively being AF31 queue, AF32 queue and AF33 queue, it will enter into the AF3 queue and be assured to transmitted.)

AF4 is a special name when DSCP is 34 or 36 or 38 in CS1. (If tabbed as 34, 36 and 38, respectively being AF41 queue, AF42 queue and AF43 queue, it will enter into the AF4 queue and be assured to transmitted.)

PS: CS7 and CS6 is silently used in protocol precedence. For example,in STP, EF is greater than AF4, AF3, AF2, AF1 and BE.

Here pay attention to one point. That is, in AF1, the abandoned level of AF31 is greater than AF12 and AF11. Why is it? Let’s review a knowledge point about the former DSCP.

AF11, being marked as 10 and written as 8-bit binary , is 001010 00.

AF12, being marked as 12 and written as 8-bit binary , is 001100 00.

AF13, being marked as 14 and written as 8-bit binary , is 001110 00.

The former three bits in TOS fields are IPP, then they are D,T and R. The more number 1 are in these three places, the higher discard rate are. (D, T and R, three bit, respectively represent delaying requires, handing capacity and reliability. ) Thus, AF11 is more important than AF12 and AF13, but AF21 is more important than AF11.

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